The 3D anatomy of the fossilised Ligulalepis skull reveals previously unknown details of the pattern of dermal skull bones, the shape of the brain cavity, and other soft tissue features (such as nerves and blood vessels) in this species.

Why are we so excited about discovering the structure of an ancient fish skull? Because Ligulalepis sits in a very important position in the vertebrate evolutionary tree.

Boney fishes – and us

Fishes are the most diverse group of backboned animals (vertebrates) on the planet, with roughly 30,000 known species.

The vast majority of these (around 98 percent!) are bony fishes or osteichthyans.

These includes most of fishes we like to eat, such as salmon, tuna and trout, as well as fishes we keep as pets, like goldfishes and guppies.

Both the old and new skulls were the focus of our new paper. Our team first prepared the tiny more recent specimen (less than 2cm in length) out of the rock using using weak acetic acid to expose the bone, as the carbonate rock dissolved.

Then we used micro computed tomography (CT) scanning to visualise the skeletal anatomy of the two known Ligulalepis specimens. Powerful X-rays pass through the bones to reveal many hidden features inside the skulls.

The scans revealed a perplexing mixture of characters in the skulls of these fish. Some features, like the shape of the inner ear canals, seemed to belong to cartilaginous fishes such as sharks. Other features, like the overall shape of the brain case, were clearly osteichthyan (bony fish) characters.

The pattern of bones that form the skull roof was an unexpectedly primitive feature also seen in an extinct group of jawed fishes called placoderms.

The CT technique enabled us to reconstruct what the brain cavity of this 400 million year old fish looked like, allowing us to digitally restore the brain shape for the first time (see video below).

Its unique position on the tree of life means that Ligulalepis provides great insight into what the ancestor of the two major radiations of bony fishes looked like.

Furthermore, the detailed analysis of its newly revealed characters helps to clarify the early evolutionary radiation of all animals with a bony skeleton – including us humans.

The position of Ligulalepis in the evolutionary family tree at the base of the osteichthyan radiation. (Brian Choo)

Missing fossils

Most modern fish are ray-finned fish (actinopterygians), with around 29,000 living species. Their origins can be confidently dated back to the Middle Devonian, around 390 million years ago with fishes like Cheirolepis from Scotland.

However the existence of the related group “lobe-fins” (sarcopterygians) is much older, with early representatives of that group such as Guiyu, from China, dated at about 430 million years ago. This suggests there are some parts of the early fish fossil record that are poorly understood.

Our fossil and the new analyses we have performed helps to resolve the big question about what the ancestor of all modern bony fishes looked like. It also illustrates the sequence of changes these early fish went through to achieve their modern “body plan”.

Also, our analyses show that Ligulalepis was the closest known species to a peculiar group of early lobe-finned fish called “psarolepids” known only from China.

It supports the hypothesis that the first early boney fish originated and radiated in China, then migrated out via East Gondwana (Australia and Antarctica), the part of the southern supercontinent closest to the ancient Chinese terrains.

Our findings highlight that the evolutionary family tree of the first bony fishes is much more complicated than we had thought, demonstrating the importance of palaeontology to help us more accurately understand our distant origins.